Fuel cells having a structure in which an anode electrode and a cathode electrode face each other across a membrane having ion conductivity have been known as, for example, polymer electrolyte fuel cells. In general, fuel cells have a layered structure in which an anode electrode is disposed on one surface of a membrane having ion conductivity (e.g., an electrolyte membrane comprising an ion-exchange resin) and a cathode electrode is disposed on the other surface thereof.
A fuel (e.g., hydrogen) is supplied to the anode electrode and then protons (H+) are obtained as a result of catalysis while two electrons (e−) are released toward the cathode electrode. Protons produced at the anode electrode pass through the membrane having ion conductivity to reach the cathode electrode. Then, as a result of catalysis, protons receive two electrons (e−) released from the anode electrode and, together with oxygen ions generated from externally supplied oxygen, form water. Electrons migrate through an external circuit, producing an electric current.
Specifically, the reaction represented by H2→2H++2e− takes place on the anode side and the reaction represented by 2H++½O2+2e−→H2O takes place on the cathode side. That is, the overall reaction represented by H2+½O2→H2O takes place for power generation. In order for chemical reactions to proceed with good efficiency, catalysts are used for electrodes as described above. For example, platinum is often used in polymer electrolyte fuel cells.
In recent years, fuel cells (biofuel cells) in which desired reactions are allowed to exclusively proceed with the use of enzymes as catalysts have been suggested. In these biofuel cells, a fuel is decomposed using enzymes that function as catalysts so as to be separated into protons and electrons. Fuels containing alcohol such as methanol or ethanol, a monosaccharide such as glucose, or a polysaccharide such as starch are used for such biofuel cells that have been developed.
Non-Patent Document 1 and Patent Document 1 disclose biofuel cells in which an electron transfer mediator is immobilized on an electrode. Polyvinylimidazole is used as a solidifying agent for immobilizing an electron transfer mediator on an electrode material for the biofuel cells disclosed in Non-Patent Document 1 and Patent Document 1.
In addition, Patent Document 2 discloses a fuel cell having a structure in which a positive electrode and a negative electrode face each other across an electrolyte containing a buffer substance, an immobilized enzyme is used for either one or both of the positive electrode and the negative electrode, and a compound containing an imidazole ring is used as the buffer substance. The fuel cell disclosed in Patent Document 2 is produced by immobilizing bilirubin oxidase on a positive electrode and, when an electrolyte containing an imidazole buffer solution is used, a high current density is achieved for unipolar evaluation of a cathode electrode.
Further, Patent Document 3 discloses a sensor for measuring a blood sugar level using an enzyme and an electron receptor. The sensor disclosed in Patent Document 3 includes a heterocyclic compound such as imidazole as well as an enzyme and an electron receptor so as to improve preservation stability of the enzyme (for suppression of changes in the current value before, during, and after preservation).
Meanwhile, fuel cells in which an anode electrode does not have a catalytic function and power generation is achieved through direct oxidization of fuel itself have been known, as described above. Specifically, it is known that fuel cells in which a reductant such as an aqueous solution of ascorbic acid or the like is used as a fuel and an anode electrode does not have a catalytic function can generate power at ordinary temperature and ordinary pressure through oxidation of the reductant (Non-Patent Document 2).